1. Field of the Invention
The present invention relates to a piezoelectric actuator, that utilizes a reverse piezoelectric effect and an electrostrictive effect under the influence of a large electric field, such as a laminated actuator, a piezoelectric transformer, an ultrasonic motor, a bimorph cell, a sonar, a piezoelectric ultrasonic transducer, a piezoelectric buzzer, or a piezoelectric loudspeaker.
2. Description of the Related Art
Piezoelectric actuators utilizing piezoelectric ceramic materials are industrial products that convert electric energy into mechanical energy by making the most of a displacement attributable to a reverse piezoelectric effect, and are widely applied to the fields of electronics and mechatronics.
The piezoelectric actuator includes: a piezoelectric element that has at least has one pair of electrodes formed on a sheet of piezoelectric ceramic; a holding part that holds the piezoelectric element; an adhesive member or a constraining member such as a spring that constrains the piezoelectric element to stay in the holding part; a lead via which a voltage is applied to the piezoelectric element and an electrical insulation member such as a resin or silicone oil that is coated over the pair of electrodes. In the piezoelectric actuator, the piezoelectric element including the sheet of piezoelectric ceramic is constrained by means of adhesion, molding, or a spring. Although no voltage is applied, a mechanical force of constraint (presetting load) is applied. Moreover, in the piezoelectric actuator, when a voltage is applied to the piezoelectric actuator, the piezoelectric element is displaced along with a rise in the voltage. This increases the mechanical force of constraint (increases the load).
Consequently, the displacement of the piezoelectric actuator, unlike the displacement performance of the piezoelectric element itself, is smaller due to a presetting load and an increase in the load.
The working conditions and driving conditions for the piezoelectric actuator include such parameters as the temperature, driving electric field strength, driving waveform, driving frequency, and whether a driving mode is continuous driving or intermittent driving. A temperature range varies greatly depending on the ambient temperature for use of products. The lower limit of the temperature range is equal to or higher than −40° C. and the upper limit thereof is equal to or lower than about 160° C. For a piezoelectric buzzer, a sonar, a piezoelectric loudspeaker, or the like, the amplitude in driving electric field strength is equal to or smaller than 500 V/mm. For an ultrasonic motor, a piezoelectric transformer, or the like, the amplitude is equal to or smaller than 1000 V/mm. For a laminated actuator, the amplitude is equal to or smaller than 3000 V/mm. Moreover, when resonant driving is adopted as a driving form, the driving waveform is a sine wave. For the other driving forms, the driving waveform may be any of various waves, that is, the sine wave, a trapezoidal wave, a triangular wave, a rectangular wave, and a pulsating wave. Moreover, the driving frequency employed for the ultrasonic motor, sonar, piezoelectric ultrasonic transducer, or the like is equal to or higher than 20 kHz, while the driving frequency employed for the other products falls below 20 kHz.
As the piezoelectric ceramic employed in the piezoelectric actuator, for example, a lead zirconium titanate (Pb(Zr,Ti)O3) system (hereinafter a PZT system) or the like has been adopted. The PZT-system piezoelectric ceramic exhibits high piezoelectric properties, and is used for a majority of piezoelectric ceramics that are currently used in practice. However, as the PZT-system piezoelectric ceramic contains lead oxide (PbO) exhibiting a high vapor pressure, the load it imposes on an environment is large.
Various piezoelectric ceramics of a barium titanate (BaTiO3) system containing no lead have been developed.
Specifically, for example, Japanese Unexamined Patent Publication No. 11-180766 has disclosed a composition whose piezoelectric strain constant d33 measured according to a resonance method is equal to or larger than 300 pC/N. The rate of temperature-dependent change in the piezoelectric strain constant d33 is small at the temperatures ranging from −30° C. to 85° C.
Japanese Unexamined Patent Publication No. 2003-128460 has disclosed a piezoelectric element that includes a sheet of a barium titanate system piezoelectric ceramic and an electrode. Herein, the rate of temperature-dependent change in a piezoelectric strain constant d31 calculated from a distortion factor exhibited by the element with a signal having an amplitude in electric field strength of 1 kV/mm applied thereto is small.
However, the piezoelectric actuator including the conventional piezoelectric element is not strong enough to withstand practical use.
The piezoelectric actuator is requested to exhibit various characteristics according to the usages of diverse industrial products, such as, a laminated actuator, a piezoelectric transformer, an ultrasonic motor, a bimorph cell, a sonar, a piezoelectric ultrasonic transducer, a piezoelectric buzzer, and a piezoelectric loudspeaker. However, in reality, no piezoelectric actuator fully satisfies the characteristics requested by the diverse industrial products such as the laminated actuator, piezoelectric transformer, ultrasonic motor, bimorph cell, sonar, piezoelectric buzzer, and piezoelectric loudspeaker. Further improvements have been demanded.